Evolution of Ultra-Fine Grains in AZ31 and AZ61 Mg Alloys during Multi Directional Forging and Their Properties

Abstract

AZ31 and AZ61 Mg alloys were multi directionally forged (MDFed) during decreasing temperature from 623 K to 423 K to cumulative strain of ΣΔε=4.8 by Δε=0.8 pass strain at a strain rate of 3×10⁻³ s⁻¹. In both Mg alloys, the average grain size gradually decreased with increasing cumulative strain. After straining to ΣΔε=3.2, i.e., after 4 passes of MDF, ultra fine grained (UFG) microstructures with average grain size of 1 μm were uniformly evolved. By prolonged straining, the grains became further finer. The AZ61 Mg alloy MDFed to ΣΔε=4.0 showed quite high hardness over 1.2 GPa, while that of the AZ31 Mg alloy was 850 MPa at ΣΔε=4.8. The differences of UFG evolution and mechanical behaviors during MDF of AZ 31 and AZ61 Mg alloys are precisely investigated.